Picture transmitting system



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PICTURE TRANSMITTING SYSTEM Original Filed Nov; 30, 1934 mwa. www.

Sem' w, 3939 A. v. BEDFORD 2,3?946 PICTURE TRANSMITTINQ SYSTE5- V original Filed'Nov. 3o, 1954' '3 sheetsheet 2 Sept. w, ma A, v BEDFORD' www PICTURE IRNSMITTING SYSTEM Original Filed Nov. 30, v19254 3 Sheets sheet 3 voL 'rs 0 TIME y TIME i INVENTR HTTORNEY Patented sept'. 19, 1939 UNITED STATES PATENT OFFICE PICTURE TRANSMITTING SYSTEM Alda V. Bedford, Collingswood, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, Delaware N. Y., a corporation of 17 Claims.

My invention relates to picture transmitting systems and particularly to systems of the type employing a cathode-ray transmitter tube.

In a preferred type of cathode-ray transmitter 5 tube, a mosaic of light-sensitive elements is positioned at an acute angle with respect to the axis of the tube whereby an optical image may be projected upon the same mosaic surface that is to be scanned by the electron beam'. Because of this design, the electron beam scans a keystone shaped area on the mosaic, instead of a rectangular area, unless a correction is made in the horizontal deilection, with the result that the picture appearing at the receiver is distorted.

When employingthe above-described type of tube, a correction must be made in the vertical deflection, also, to avoid a distorted picture at the receiver. This vertical correction, like the horizontal correction, is necessitated by the position of the mosaic with respect to the tube axis.

It is, accordingly, an object of my invention to provide a picture transmitting system of the above-mentioned type which will transmit an undistorted picture.

A further object of my invention is to provide an improved method of and means for correcting keystone error in a cathode-ray tube of the abovementioned type.

A still further object of my invention is to provide means for correcting errors in the vertical deflection in a cathode-ray tube oi. the abovementioned type.

Other objects, features and advantages of my invention will appear from the following descrip- 35 tion taken in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of a television transmitter embodying my invention,

Fig. 2 is a simplified plan view of the cathodel ray transmitter tube shown in Fig. 1,

Fig. 3 is a similar side view of the abovementioned cathode-ray tube,

Fig. 4 is a circuit diagram showing a modication of a portion of the circuit shown in Fig. 1,

Figs. 5 to 10 are curves which are referred to in explaining the keystone correction feature of my invention, and

Figs. 11 and 12 are curves which are referred to in explaining the vertical deflection correcting feature of my invention,

(Cl. P18-'1.2)

Referring to Fig. 1, the television transmitter includes a cathode-ray transmitter tube I of the type which is described in an article by V. K. Zworykin, published in the January, 1934, issue of the Proceedings of the Institute of Radio En- 5 gineers. This tube includes an electron gun consisting of a cathode 3, a control electrode 5 and a first anode l. A second anode 9 is provided which, in addition to accelerating the electrons, aids in the electrostatic focusing of the electron 1g beam. 'I'he second anode also functions as a collector of photo-electrons which are emitted from a mosaic I I of light-sensitive elements.

'I'he light-sensitive elementsv of the mosaic Il are insulated from each other and from a metal- 15 lic back plate, as described in the above-menl tioned publication.

An optical image of the object I3 to be transmitted is projected upon the mosaic II by means of a suitable optical system I5. In order that the 20 light-sensitive surface of the mosaic I I may have the optical image projected thereon and, at the same time, be scanned by the electron beam, it is positioned at an acute angle to the longitudinal axis of the cathode-ray tube. More spe- 25 ciflcally, it is positioned at an acute angle to the axis of the electron gun.

'Ihe cathode-ray tube I is provided with suitable deiiecting devices, such as defiecting plates I1 and deecting coils I9, for deecting the elec- 30 tron beam both horizontally and vertically to sc'an the mosaic I I. If an optical image is projected upon the mosaic, picture signals are developed during the scanning of the mosaic by the electron beam, these picture signals appearing across a resistor 2l which is connected between the mosaic back plate and ground. These picture signals are supplied to a suitable transmitter 23 or to a transmission line 1" or transmission to a receiver.

Because of the angular position of the mosaic II with respect to the electron gun, certain corrections must be made in both the horizontal and vertical deflection of the electron beam. That such corrections are necessary will be apparent by referring to Figs. 2 and 3, like parts in these figures and Fig. 1 being indicated'y by the same reference numerals. In Fig. 2 the dotted lines 25 .indicate the limits of the horizontal deiiection of -the electron beam. It will be evident that when does when it is striking the upper edge of the l mosaic, providing the angle between the limits of the horizontal deflection remains constant as the beam is deflected upwardly. As a result. the beam scans a keystone shaped area on thermosaic that is indicated by the area enclosed in dotted lines 21. One of the main objects of-my invention is to so correct the horizontal deflection that a rectangular, rather than a keystone, shaped area is scanned on the mosaic.

In Fig. 3 the dotted lines indicate several positions of the electron beam as it is deecting upwardly on the mosaic. From inspection of this diagram it will be apparent that, for a given angular upward deflection of the electron beam, it traverses a longer path on the mosaic at the top thereof than it does at the bottom. As a result,

unless a suitable correction is made, the picture appearing atthe receiver will be crowded together in the lower region of the picture.

Referring now to the deflecting circuits shown in Fig. 1, the vertical deecting circuit for supplying a saw-tooth current to the deflecting coils I9 is indicated generally at 29, while the hori-` zontal de'c'ting circuit for supplying a saw-tooth voltage to the deecting plates |1 is'indicated generally at 3l.

Both deflecting circuits are controlled by an impulse generator 33 which generates horizontal deiiecting impulses at a comparatively high frequency and vertical deiiecting impulses at a comparatively low frequency. In the particular embodiment illustrated the horizontal deilecting impulses occur at the rate of 10,290 per second, while the vertical deflectingl impulses occur at the rate ofY 60 per second. The impulse generator 33 may be of any suitable type, one form comprising a disc driven by a synchronous motor, the disc containing openings therein through which light is projected upon a photo-electric cell. An impulse generator of this type is shown in French Patent No. 742,671, dated January 4, 1933, and, as applied to interlaced scanning, in Ballard application Serial No. 623,325, illed July 19, 1932, and assigned to the Radio Corporation of America. While the frequencies of the deiiecting impulses given above are for a system employing interlaced scanning, it will be understood that my invention is not limited t'o such a system. The vertical deflecting circuit includes an ampliertube 35, a discharge tube 31, and an output tube 39. The particular amplifier tube 35 illustrated is of the pentode type having a cathode 4|, a. control grid 4,3, a screen grid 45, a suppressor grid 41 and a plate'49. The control grid 43 is provided with a suitable bias potential by means of the usual. self-bias resistor 5| which is shunted by a by-pass condenser 53. The screen grid 45 and plate 49 are supplied with positive potentials from a suitable unidirectional plate or B voltage supply (not shown) through filter resistors 55 and 51, respectively. Suitable by-Pass or filter condensers 59 vand 6| are provided in the screen grid and plate circuits.

The vertical defiecting impulses are impressed As is well known, in ordery to produce a ow of current through the deilecting coils |9 which has a good saw-tooth wave form, it is necessary that the voltage impressed across the coils contain a saw-tooth component in addition to the impulse component which would be required if the defiecting coil circuit were purely inductive. 'Ihis saw-tooth component is provided by charging a condenser 13 from the plate voltage supply through a highly resistive circuit which includes a filter resistor 15, a variable resistor 11 and a iixed resistor 19. This charging circuit alsov includes another variable resistor 8|, which has a comparatively low resistance value, for supplying the'necessary voltage impulse component: positive potential for the plate 1| of the discharge tube 31 is supplied through the filter resistor 15 and theresistors 11 and 19.

In operation, the vertical deflecting impulses which are supplied from the impulse generator 33 are given a negative polarity whereby they appear in the input circuit of the discharge tube 31 as impulses of positive polarity. The grid 69 of 'the discharge tube 31 normally is suiliciently negative to substantially block plate current flow between the deecting impulses. The condenser 13, which will be referred to as the vertical saw-tooth condenser, is charged at a substantially constant rate through the high impedance circuit 15-11- 19, whereby the voltage across the condenser 13 gradually increases in value. As soon as a positive impulse appears on the control grid 69, the discharge tube 31 becomes highly conductive, whereby the condenser 13 discharges very rapidly through the resistor 8| and the discharge tube. This discharge through the resistor 8| gives the desired impulse component to the voltage wave supplied to the input circuit of the tube 39. The charge and discharge of the condenser 13 produces a saw-tooth voltage wave thereacross.

The combined saw-tooth component and impulse component are impressed through a coupling condenser 83 and a resistor 85 upon the input electrodes of the tube 39. v

The output tube 39 may be of the pentode type comprising a cathode 81, the control grid 89, a

screen grid 9|, a suppressor grid 93 and a plate 95. The tube is provided with the usual selfbias resistor 91 and by-pass condenser 99. The screen grid 9| is supplied with positive potential through a filter resistor |0I, while the plate 95 is supplied with positive potential through a choke coil |03 and a resistor |05. 'Ihe choke coil |03 is provided mainly for the purpose of maintaining vthe amplification at a proper level for high frequency components of the wave.

The input circuit of the output tube 39 includes a resistor |01 and a condenser |09 connected in series, the condenser |09 being shunted by a variable resistor The purpose of the condenser |09 and variable resistor |01 is to control the wave shape of the current owing through the vertical deilecting coils 9, whereby the abovementioned correction may be made for the vertical deection. This feature of the invention will be explained in detail hereinafter. The output circuit of thevoutput tube 39 is coupled tc the deflecting coils I9 through a suitable coupling condenser H3.

Beierring now to the horizontal deilecting cir- TheV cuit 3|, it includes an ampliiler tube I5, a discharge tube 1, a second amplifier tube I|9 and an output tube |2|. The amplier tube ||5 may be of the pentode type comprising the usual cathode, control grid, screen grid, suppressor grid and plate. It is provided with a suitable negative bias by means of a self-biasing resistor |23. 'I'he plate is supplied with a positive potential through a conductor |25 and through a plate resistor |21. The screen grid is supplied with positive potential through the conductor |25 and through a iilter resistor |29, a "b1eeder resistor |28 being connected between the screen grid and ground.

The horizontal deilecting impulses are impressed upon the input circuit of the amplifier 5 through a resistor |3| and a coupling condenser |33. The amplified impulses are impressed through a coupling condenser |35 upon the input -circuit of thedischarge tube ||1.

The discharge tube H1 is of the screen grid type, the tube illustrated being a pentode comprising a cathode |31, a control grid |39, a screen grid Hi, a suppressor grid |43 and a plate |55. The tube ||1 is biased at a suitable negative potential by means of a self-biasing resistor |41. The plate |45 is supplied with positive potential through the conductor |25, a lter resistor |49, a buffer resistor |5|, and a plate resistor |53. -Itl will be noted that the resistors |41 and |49 are provided with by-pass con--V densers.

The screen grid lll is supplied with positive potential'through the conductor |25 and through a resistor |55. The resistor |55, together with a resistor |51 connected between the screen grid MI and ground, forms a potentiometer connected across the plate voltage supply for lowering the screen grid voltage to the desired value. The resistors |51 and |55 also function as a grid leak resistor for the screen grid.

In addition to the steady voltages supplied to the plate and screen grid through the abovedescribed circuits, saw-tooth voltages are impressed upon the plate |45 and the screen grid |4| through acircuit which will be described later.

In order to produce the saw-tooth voltage which isto be applied to the deflecting plates I1, a condenser |59 is provided which is charged through the three resistors |49, |5| and |53. The condenser- |59 is discharged periodically through the discharge tube ||1 to produce a saw-tooth voltag wave across the condenser |59 in the same manner as described in connection with the vertical deilecting circuit.

The saw-tooth voltage is impressed through a coupling condenser |6| upon the input circuit of the amplifier tube |19. This tube may be of the pentode type, comprising the usual cathode,

control grid, screen grid, suppressor grid, and plate. A ositive potential is supplied to the screen gri through the conductor |25 and a filter resistor |63. A positive potential is supplied to the plate through the conductor |25, the lter resistor |63, a variable wave-shaping resistor |65, and a plate resistor |61.

In order. to permit a control oi the deilecting voltage wave shape, a. condenser |69 is connected in series with :the `plate resistor |51 and ground whereby the amplitude of low frequencies is exaggerated. The extent to which the lower frequencies are amplified more than the higher frequencies isdetermined by the extent to which signals are shunted around the condenser |69 by the variable resistor 55.

The amplified saw-tooth wave which appears across the resistor-condenser combination |61- |69 is impressed upon the input circuit of the output tube |2|. In the embodiment illustrated,

the output tube 2| is of the three-electrode,

type comprising of a cathode |1|. a control grid |13, and a plate |15. The input circuit of the tube |2| includes a resistor |11 and a resistor |19 to which the control grid |13 is connected through a variable tap for the purpose of controlling the amplitude of the deecting voltage wave. The resistors |11 and |19 are shunted by a small capacity condenser |8| for the purpose of ltering out the highest frequencies in the saw-tooth wave which might produce undesired resonant eifects in the transformers |83 and |85 which couple the deecting plates |1 to the output circuit of the tube |2|. It will be noted that the tube |2| is provided with the desired negative bias by means of a self-biasing resistor |81, while the plate potential is supplied through a filter resistor |89 and the primary of the output transformer |83. The output transformer couples the output circuit of the tube |2| to a. cable, indicated by the dotted lines l9|, which extends from the horizontal deiecting circuit to the cathode-ray tube I. This transformer steps down the voltage to a value low enough to make the shunting effect of the distributed capacity of the cable negligible.

The auto-transformer |85, which is located in the immediate vicinity of the cathode-ray tube I, is provided for stepping up the dellecting voltage to the high value required for deecting the of the circuit, it includes an amplifier tube |93 which may be of the pentode type as illustrated.V The input circuit of the tube |93 is connected across the saw-tooth wave condenser 13 in the vertical deilecting circuit by means of a coupling condenser |95 and a connection through ground.

A positive potential is supplied to the plate of the pentode |93 from the plate supply through the conductor |25 and resistors |91 and |99 which are connected in parallel. A resistor 20| is included in the circuit to lower the plate voltage to the desired value. Resistors |91 and |99 are provided with a by-pass condenser 203.

It will be apparent that a substantially pure saw-tooth wave appears across the resistors |91 and |99 and that this wave is 180 out of phase with respect to the saw-tooth wave appearing across the condenser 13, the phase having been reversed by the amplifier tube |9|. The sawtooth wave appearing across the resistors |91 and |99 is shown in Figures 8 and 10.

In accordance with my invention, this sawtooth wave is utilized for modulating the voltages applied to the plate |45 and screen grid |4| of the horizontal circuit discharge tube |1 whereby the amplitude of the horizontal deflection is gradually decreased as the electron beam is deflected upwardly, the electron beam always being deected substantially the same amount to the right as to the left of the center of the mosaic.

In order to modulate the plate voltage, the point intermediate the two resistors |5| and |53 is connected through a coupling condenser 295 and a variable tap to a point on the resistor |91.

The plate modulation is produced by the ilow of vertical saw-tooth current through the. plate resistor |53 whereby a voltage corresponding to this current is added to the steady voltagel provided by the plate supply.

The screen grid |4| of the discharge tube ||1 is connected through a coupling condenser 201 to the plate end of the resistors |91 and |99 whereby a vertical saw-tooth voltage modulates the screen grid voltage due to the ilow of vertical saw-tooth current through the screen grid resistor |51.

The relative values of the plate modulating voltage and the screen grid modulating voltage may be adjusted by means of the variable tap on the resistor |91. The values of both modulating voltages may be adjusted simultaneously after the desired balance has been obtained by adjusting the value of the resistor |99 which is made variable for this purpose. It will be seen that, as the electron beam is deflected upwardly, the keystone correcting circuit causes a gradual reduction in the voltages on the plate and screen grid of the discharge tube ||1 since the vertical saw-tooth wave is decreasing in value as indicated in Fig. 8. The operation of the circuit will be better understood by referring to Figures 5 to 10.

In Fig. 5 there is shown a series of charging curves for the saw-tooth condenser |59 in the horizontal defiecting circuit. The upper curve 209 shows how the voltage increases across the condenser |59 as it is charged through the resistors |49, |5| and |53 with a certain voltage applied to the condenser-resistor combination. The lower curves 2||, 2|3, 2|5 and 2|1 show how the voltage across the condenser |59 builds up for lower voltages applied to the same condenser-resistor circuit.

The curves in Fig. 6 show how the voltage across the condenser |59 varies as the said condenser discharges through the discharge' tube ||1 with diierent voltages applied to the discharge tube screen grid |4|. charge curve 2|9 represents the case where a comparatively high positive voltage is applied to the screen grid |4|. The other curves 22|, 223, 225 and 221 show how the condenser |59 discharges less rapidly as the positive voltage on the screen grid |4| is decreased.

Referring to Fig. 7, there are illustrated two saw-tooth curves. 'I'he upper curve 229 is the one which would be generated with the keystone correcting circuit omitted, that is, with no modulation on the plate-and screen grid of the discharge tube ||1. This curve has been constructed by means of the top curve 209 in Fig. 5 and the top curve 2|9 in Fig. 6. It will be seen that the condenser |59 charges gradually along the curve 269 during the time Ti and discharges suddenly along the curve 2|9 during the time T2. It will be understood that during the time Ti the discharge tube ||1 is biased beyond cut-off, while during the time T2 a positive synchronizing impulse is being impressed upon the control grid of the discharge tube ||1 to render ithighly conductive. Only a small number of horizontal saw-teeth for each vertical saw-tooth has been indicated in Figs. 7 and 8 in order to simplify the drawings.

If an attempt is made to gradually reduce the amplitude of the horizontal saw-tooth wave by modulating only the plate voltage of the discharge tube ||1, the resulting saw-tooth has the appearance of the lower saw-tooth wave 23|.

The steepest ds- 'I'his saw-tooth wave has been constructed from the several curves in Fig. 5, since it is assumed that the charging voltage is being reduced gradually bythe saw-tooth voltage indicated4 in Fig. 8, and from the top discharge curve 2|9 in Fig. 6. It has been assumed that the discharge rate of the condenser- |53 has not changed, even though the plate" voltage of the discharge tube ||1 has been lowered, because, for all practical purposes, thisis true for tubes of the screen grid type. Since the discharge rate has remained the same, the average value of the voltage across the condenser |59 has decreased in value as indicated by the saw-tooth curve. Obviously a saw-tooth wave of this character would produce a distorted picture. Specifically, it would produce a picture which would not have vertical sides. L

Ii a three element vacuum tube were employed in place of the screen grid tube ||1, the effect illustrated by the saw-tooth curve 23| would not be so pronounced since the discharge rate of the condenser |59 would change appreciably4 with a change in plate voltage. For this reason, by utilizing a triode discharge tube with modulation on its plate only, a keystone correction may be obtained which is appreciably better than no correction, but the correction will be imperfect.

Fig. 9 illustrates the keystone correction that is obtained by means of the circuit illustrated in Fig. 1. The saw-tooth curve 233 illustrated in this figure has been constructed by means of the charging curves shown in Fig. 5 and the discharge curves shown in Fig. 6, since the charging voltage and the screen grid voltage are being reduced gradually by the saw-tooth voltage indicated in Fig. 10. It is evident that by decreasing the discharge rate of the condenser |59 at the same time that its charging rate is decreased, it is possible to maintain the average voltage across said condenser substantially constant. 'In other words, the voltage level of the alternating current axis of the saw-tooth wave curve 233 is maintained constant. Considering `the curves 23| and 233 in Figs. 7 and 9, respectively.' from a slightly dierent viewpoint, the saw-tooth curve 23| is objectionable because it .contains a strong vertical saw-tooth component whereas the saw-tooth curve 233 contains no such component. It has been found from experience that my improved keystone correcting circuit is reliable in operation and that it gives a practically perfect correction of the keystone effect.

Y Referring now to the previously mentioned correcting circuit for the Vertical deection error illustrated in Fig. 3, I correct for this error by causing the vertical deflecting saw-tooth wave to bend over as will be understood by referring to Fig. 11.

Referring to Fig. 11, if it were not for the fact that the mosaic is at an angle with respect to the electron gun, for the vertical deflection, a perfect saw-tooth current wave would be desired as indicated at 235. As previously pointed out, however, it is desired to slow down the deiiectionl of the electron beam as it is deilected upwardly. yThe saw-tooth may be bent over with a resulting slowing down'of the electron beam by eliminating some of the low frequency components of the saw-tooth wave 235, the sawtooth wave 235 lbeing the one which appears across the saw-tooth condenser 13. In the vertical deilecting circuit illustrated in Fig. 1, the coupling condenser ||3 and the by-pass con- 'amines denser 99 are deliberately made small enough in capacity to eliminate enough low-frequencies Yfor to occupy almost anyv position between the two proaches the bottom edge of the mosaic.

saw-tooth curves 231 and 235.

The curves in Fig. 11 represent both the ow of current through the vertical deiecting coils and the saw-tooth component of the voltage impressed upon the input electrodes of the'` amplier. The wave shape of the vpltage impressed upon the input electrodes of the amplier 39 is indicated by the curve in Fig. 12. It willbe seen that this voltage includes the necessary impulse component for producing the iiow of saw-tooth current through deflecting coils. Y

In Fig. 4 there is illustrated another circuit for controlling the curvature, of the vertical sawv tooth wave. Like parts in Figs. 1 and 4 are indicated bythe same reference numerals. In the circuit of Fig. 4, the condenser |09 and resistors |01 and ill of Fig. 1 have been replaced by a variable grid leak resistor 2li. Also, the condensers I I3 and 99 in the circuit have been given a capacity suicient to pass the W frequency components of the saw-tooth wave eiciently. The curvature of the saw-tooth wave is controlled by adjusting the grid leak resistor 24H until the low frequency components are attenuated to the desired degree.

In some cases it may be preferred to deflect the electron beam from the edge of the'mosaic farther from the electron gun to the edge of the mosaic nearer the gun, that is, from top to bottom in the cathode-ray tube illustrated. It will be apparent that, in such a. case, the speed of vertical deflection should be increased as the beam ap- This may be accomplished by so adjusting the grid leak resistor Iii of Fig. 1 that the saw-,tooth wave bends upward above the line of a perfect saw-tooth, instead of downward. I

Also, if the beam is deflected downwardly, the

amplitude of the horizontal deflecting saw-tooth must be increased as the beam approaches the bottom edge of the mosaic, this being accomplished merely by applying a vertical deflecting saw-tooth wave to the discharge tube II1 which is 180 degrees out of phase to that assumed in describing Fig. 1. In other words, both the charging voltage and the positive screen grid voltage are increased as the beam is deflected downwardly. The change in amplitude of the charging voltage and in the amplitude of the positive mosaic ispositioned at an screen grid voltage is always in the same sense, that is, the charging voltage and the screen grid voltage either increase together or decrease. together.

From the foregoing description it will be seen that I have provided correcting circuits which enable an operator to so control the horizontal and vertical deflections of the transmitter tube electron -beam that an undistorted picture istransmitted even though the transmitter tube angle with respect to the electron gun.

In the embodiment of my invention illustrated in Fig. 1, certain of the circuit values were as l follows:

In the vertical deecting circuiti Resistor 70:4 megohms Resistor 79 :400,000 ohms Resistor 77:1 megohm Resistor 81:5,000 ohms Condenser 73:.5 micro-farads Condenser 83:.02 micro-farads Resistor 85:.5 megohms Resistor 107:.5 megohms Resistor 111:.7 megohms Condenser 109:.01 micro-farads In the horizontal deflecting circuit:

'Iransformer 183:5 to 1 step down ratio Transformer 185:15 to 1 step up ratio Other circuit values:

Resistor 201: 1,000 ohms Resistor 197 :25,000 ohms Resistor 199:5,000 ohms Condenser 207 :4 micro-farads Condenser t 205:4 micro-farads Condenser 203 :.01 micro-farads It may be noted that the variable resistors are usually set at a value equal to about one-half the resistances values given for them in the above table.

It will be understood that various other modirlcations -may be made in my invention without departing from the spirit land scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

l. In a cathode-ray tube deflectlng circuit, means for producing a saw-tooth wave occurring at a relatively low frequency, means for gradually charging and suddenly discharging a condenser at a relatively high frequency to produce a saw-tooth wave occurring at said high frequency, and means for varying said charge and said discharge in accordance with said low frequency saw-tooth wave, said charge and discharge being so related that the average voltage across said condenser remains substantially constant.

2. In apparatus for producing a modulated saw-tooth wave. a condenser, means for charging said condenser at approximately a constant rate during a certain period, means for discharg. ing said condenser suddenly at the end of said period, means for charging said condenser again for again suddenly discharging said condenser at a rate which differs from said preceding discharge rate by an amount such as to maintain the average voltage across said condenser substantially constant.

3. In combination, a cathode-ray tube comprising an electron gun and a mosaic of lightsensitive elements positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, deflecting means for deflecting said electron beam vertically, derlecting means for deiiecting said electron beam horizontally, means for producing a saw-tooth Wave occurring at a relatively low frequency and for applying it to said first deecting means, and means for producing a saw-tooth wave occurring at a relatively high frequency and for applying it Vto said second deflecting means, said last means comprising means for charging a condenser successively at a changing charging rate determined by' said low frequency saw-tooth wave, an electric discharge tube of the screen grid type, and means for discharging said condenser successively through said screen grid tube at a changing discharging rate determined by said low frequency saw-tooth wave.

4. In apparatus for producing a modulated saw-tooth wave, a condenser, a source of charging voltage and an impedance unit having a high resistance value connected in series with said condenser whereby the condenser charges at an approximately constant rate for a given value of said voltage, a screen grid tube having a control grid and a screen grid and having its cathodeanode space path connected across said condenser, means for maintaining said control grid at a negative potential whereby said condenser charges at said constant rate for a certain period, means for periodically decreasing said nega- 'tive potential sufficiently for a certain period to cause said condenser to discharge suddenly through said tube, means for changing the positive potential on said screen grid for successive discharge periods, and means for changing the value of said charging voltage for successive charging periods, the changes in said charging potential and said screen grid voltage being in the same sense.

5. In combination, a condenser, means for charging said condenser successively at approximately a constant rate during charging periods, a vacuum tube havingl a plurality of control electrodes and having main electrodes connected across said condenser, means for periodically applying a potential to one of said electrodes such that said condenser discharges through said tube at the end of each charge period, means for changing said charging rate for successive charging periods, and means for applying a gradually changing potential to the other of said control electrodes whereby the discharge rate of said condenser changes for successive periods, the change in charging and discharging rates being in the same sense.

6. In combination, a cathode-ray tube comprising an electron gun and a mosaic of lightsensitive elements positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, the top edge ofsaid mosaic being a different distance from said gun than the bottom edge thereof., a vertical deflecting device for defiecting said beam vertically from near one of said edges to near the other of said edges, a horizontal deecting device for deiiecting said beam horizon-l tally, means for producing a saw-tooth wave occurring at a relatively low frequency and for applying it to said first deilecting means, and means for producing a saw-tooth wave occurring at a relatively high frequency and for applying it to said second deflecting means,v said -last means comprising a condenser, means for charging said condenser successively at approximately a constant rate during charging periods, a vacuum tube having a plurality of control electrodes and having main electrodes connected across said condenser, means for periodically applying a voltage impulse to one of said electrodes such that said condenser discharges through said tube at the end of each charge period. said voltage impulses occurring at said relatively high frequency, means for changing said charging rate for successive charging periods in accordance with said low frequency saw-tooth wave, and means for applying said low frequency sawtooth wave to the other of said electrodes whereby the discharge rate of said condenser changes for successive periods, the change in charging and discharging rates being in the same sense.

'7. Apparatus according to claim 6 characterized in that said vacuum tube is of the type in which the plate impedance remains substantially constant over a wide variation in potential applied between said main electrodes.

8. Apparatus according to claim 6 characterized in that said vacuum tube is of the screen grid type.

9. In combination, a cathode-ray tube comprising an electron gun and a screen structure positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, deflecting means for defiecting said beam in a certain direction in the plane of said acute angle, deflecting means for deflecting said beam at right angles to said plane, means for applying a succession of sawtooth defiecting waves to said first defiecting means, and means for so shaping each individual saw-tooth that said beam traverses the surface of said screen structure at a uniform speed in said certain. direction as it is deflected.

l0. In combination, a cathode-ray tube comprising an electron gun and a screen structure positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, the top edge of said screen structure being farther from said gun than the bottom edge thereof, a defiecting device for deilecting said beam vertically from one of said edges to the other of said edges, means for applying a deflecting wave to said deflecting device, and means for so shaping said wave that said beam traverses-a path on said screen structure in a vertical direction at a constant speed.

11. In combination, a cathode-ray tube comprising an electron gun and a screen structure positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, the top edge of said screen structure being farther from said gun than the bottom edge thereof, a deilecting device for deflecting said beam vertically from said bottom edge to said top edge, means for applying a deecting wave to said deilecting device, means for so shaping said wave that said beam is deflected at a decreasing speed as it approaches said top edge such that said screen structure is traversed in a vertical direction at a uniform speed, and means for simultaneously deiiecting said beam in a horizontal direction.

cal wave, an electron tube provided with a controlv grid and forming part of said second-named generator. a condenser and a source of charging voltage therefor forming part of said secondnamed generator, and connecting means between said generators for causing both the charging -voltage and the bias on said control grid to vary in accordance with the low-frequency wave.

13. In combination, a cathode ray tube comlprising an electron gun and a screen structure positioned in the path of the electron beam generated by said gun and also positioned at an acute vangle to the axis of said gun, deecting means for deflecting said beam in a certain dlrection in the plane of said acute angle, deflecting means for defiecting said beam at right angles to said plane, means for applying a successioniof saw-tooth deilecting waves to said first deecting means, and means for so shaping each individual saw-'tooth that said beam traverses the surface of said screen structure at a uniform speed in said certain direction as itis deflected, said last means comprising a frequency selective means for causing the attenuation of low frequency components of said saw-tooth deflecting waves according to a certain law and another frequency selective means for causing the accentuation of low frequency components of said sawtooth defiecting waves according to 'a diiferent law than said certain law. i

14. In combination, a cathode 'ray tube comprising an electron gun and screen structure positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, deflecting means for deiiecting said beam in a certain direction in the plane of said acute angle, deflecting means for deecting said beam at right angles to said plane, means for applying a succession of saw-tooth deecting waves to said first deflectingvmeans, and means for so shaping each individual saw-tooth that said beam traverses the surface of said screenstructure at a uniform speed in said certain direction as it is deected, said last means including frequency selective means for attenuating low frequency components ofl said saw-tooth waves.

15. In combination, a cathode ray tube comprising an electron gun and a screen structure positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to the axis of said gun, deecting means for defiecting said beam in a certain direction in the plane of said acute angle, deilecting means for deecting said beam at right angles to said plane, means including a coupling condenser in series in a transmission channel for applying a succession of saw-tooth deflecting waves to said flrst defiecting means, said coupling condenser having suiciently small capacity to cause a substantial loss of low frequency components in said saw-tooth waves whereby each saw-tooth wave is caused to bend over, and means including said coupling condenser for so shaping each individual saw-tooth that said beam traverses the surface of said screen structure at a substantially uniform speed in said certain direction as it is deilected, said means also including a condenser in shunt to said channel, said shunt condenser having resistance effectively in series therewith and having resistance effectively in shunt thereto whereby certain low frequency components of said sawtooth waves are accentuated.

16. In a cathode ray tube'defiecting circuit. means for producing asaw-tooth wave occurring at a relatively low frequency, means for charging and discharging a condenser at a relatively high frequency to produce a saw-tooth wave occurring at said high frequency, and means for varying the rate of said charge and means for varying the rate of said discharge in accordance with said low frequency, saw-tooth wave, said rate of charge and said rate of discharge being' so related that the average voltage across'said condenser remains substantially constant whereby the voltage level of the alternating current axis of said saw-tooth Wave is constant.

17. In a cathode ray tube deecting circuit, means for producing a saw-tooth wave occurring at a relatively low frequency, means for producing a succession of saw-tooth waves occurring at a relatively high frequency, and means for so modulating said high frequency waves by said low frequency wave that modulated high frequency waves are produced which contain substantially no low frequency saw-tooth component whereby the voltage level of the alternating current axis of said modulated high frequency waves is conetant.

AIDA V. lBEDFORD. 

